1. Field of the Invention
The invention relates to methods for charging and forwarding filaments to a web formation zone.
2. Description of the Prior Art
It is known to make nonwoven fabrics from continuous synthetic filaments by passing groups of filaments through attenuators to drive the filaments onto a foraminous belt where a nonwoven web is formed as the belt is moved past the attenuators. The web is subsequently bonded in a conventional manner to increase the strength and enhance other properties of the web. If a salable product is to be made, uniformity of the web is of paramount concern. If the filaments laid down to form the web are not properly separated the web will have a ropy, unattractive appearance which will render it unsalable.
While the turbulence of the air moving through the attenuator will tend to separate filaments from each other to some extent, it has been found that even better filament separation can be achieved by the use of tribo-electricity or by using a corona discharge system which applies an electric field and thereby an electric charge to the filament group. Several problems are encountered in charging filament groups and in controlling the distribution of a number of filament groups which may go into a web having a width of as much as several meters.
One of the problems encountered in forming a web such as described above is that of filament separation. U.S. Pat. No. 3,163,753, for example, discloses a filament charging apparatus wherein a wide layer of filaments is charged as the layer is pulled under tension over a target electrode. Since the filaments are in a layer it is necessary to use a plurality of corona discharge electrodes positioned in a row perpendicular to the direction of travel of the filaments in order to equally charge all of the filaments.
Another problem encountered in web formation is the interaction of adjacent filament bundles and adjacent corona charging systems which are necessary to form a web several meters in width. In order to have uniformity across a web several meters in width it is necessary that the filament groups being advanced to the web, and the corona charging systems associated with each group, be positioned in close proximity to each other. This can in some cases lead to undesirable electric field distortion and physical interference between adjacent filament groups. In the present invention, the filament forwarding and charging systems can be positioned in close proximity to each other without adverse effect.
U.S. Pat. No. 3,338,992 is typical of the prior art patents wherein a corona charging system is used upstream of an attenuator to apply a charge to a group of filaments, the filaments being held under sufficient tension to prevent separation of the filaments in the group before the filaments pass through the attenuator. The problem with this arrangement is that it is more difficult to obtain a high filament charge when the filaments are held together. In addition, some of the charge can be lost as the group passes through the attenuator and there is a risk of having the charged filaments stick to the inside of the attenuator. In the present invention, the corona charging system is positioned downstream of the attenuator and filaments are fed in such a manner that the filament group spreads from a bundle configuration into a fan configuration as the charge is applied. This permits a significantly higher filament charge and much reduced risk of charge loss.
The British counterpart of U.S. Pat. No. 3,338,992, British Pat. No. 932,482, suggests that a corona discharge can be used either upstream of or downstream of the attenuator but offers no explanation of how this might be done downstream of the attenuator.
U.S. Pat. No. 3,689,608 is typical of several patents which disclose apparatus for making a web from plexifilaments wherein the plexifilament is deflected and spread to fall past a target electrode to a charged belt. The deflector spreads the plexifilament above the electrode, so that it is necessary to use an array of charging needles disposed laterally across the path of the spread plexifilament in order to obtain a uniform charge across the plexifilament.